US4334244A - Adaptive image enhancement system - Google Patents
Adaptive image enhancement system Download PDFInfo
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- US4334244A US4334244A US06/173,005 US17300580A US4334244A US 4334244 A US4334244 A US 4334244A US 17300580 A US17300580 A US 17300580A US 4334244 A US4334244 A US 4334244A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N5/00—Details of television systems
- H04N5/14—Picture signal circuitry for video frequency region
- H04N5/20—Circuitry for controlling amplitude response
- H04N5/205—Circuitry for controlling amplitude response for correcting amplitude versus frequency characteristic
- H04N5/208—Circuitry for controlling amplitude response for correcting amplitude versus frequency characteristic for compensating for attenuation of high frequency components, e.g. crispening, aperture distortion correction
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N5/00—Details of television systems
- H04N5/14—Picture signal circuitry for video frequency region
- H04N5/21—Circuitry for suppressing or minimising disturbance, e.g. moiré or halo
Definitions
- This invention relates to real-time image processing systems, and more particularly, to a video signal processing system for enhancing a video image by combined noise filtering and edge sharpening.
- the present invention is directed to a video image enhancement system which adapts itself to noise filtering of the video signal in image regions of nearly uniform intensity while providing edge sharpening in a region of sharp contrast and high detail.
- the system adapts globally to the input signal-to-noise (SNR) ratio so that if if the noise level is very low, there is enhancement by edge sharpening of all detail, but if the noise level is high, noise filtering but no edge enhancement is provided.
- SNR signal-to-noise
- This global adaption to the signal-to-noise ratio is independent of picture detail.
- the system adapts locally to the gradient magnitude to provide either noise filtering or edge enhancement with a smooth transition between these two conditions.
- the video enhancement system of the present invention is based on the concepts that human observers detect the presence of noise much easier in a nearly uniform region than near an edge, while responding more to an edge brightness difference than to the same brightness difference in regions not adjacent to each other. This suggests that by dividing the images into regions of nearly uniform brightness and regions of higher contrast, it is possible to limit noise filtering to the low contrast regions and edge sharpening to the higher contrast regions. Because such segmentation requires some kind of thresholding to define the two regions, resulting in loss in spatial resolution and introduction of artifacts, the present invention provides a system which is adaptive to image content as measured by the magnitude of the picture gradient. The degree of noise filtering is decreased gradually while the degree of edge sharpening is increased gradually as the magnitude of the gradient increases. At the same time, the rate of transition is made adaptive to the input signal-to-noise ratio of the image so that more adaptive noise filtering is performed if the image is noisier and more adaptive edge enhancement is performed if the image is cleaner.
- FIG. 1 is a graphical representation of the idealized characteristics of the enhancement system
- FIG. 2 is a schematic block diagram of one embodiment of the invention.
- FIG. 3 is a graphical representation of the characteristics of the system of FIG. 2;
- FIG. 4 is a schematic block diagram of a preferred embodiment of the present invention.
- FIG. 5 is a graphical representation of the characteristics of the embodiment of FIG. 4;
- FIG. 6 is a schematic block diagram of a median filter
- FIG. 7 is a block diagram of the processor P 2 of FIG. 2;
- FIG. 8 is a block diagram of the processor P 4 of FIG. 2;
- FIG. 9 is a block diagram of the processor P 6 of FIG. 4.
- FIG. 1 there is shown the overall characteristics of a video adaptive image enhancement system having the required properties.
- the degree of average filtering to reduce noise is decreased and the degree of edge filtering increases.
- a family of curves for different levels of SNR, as indicated by ⁇ , show the characteristic shifts to provide more or less average filtering while providing less or greater edge filtering as the noise level increases or decreases.
- the degree of noise filtering decreases gradually while the degree of edge sharpening increases gradually as the magnitude of the gradient increases.
- the rate of transition is made adaptive to the input signal-to-noise ratio of the image.
- the curve ⁇ 1 represents an image generated from a video signal of relatively low SNR while the curve ⁇ 2 corresponds to a video signal of higher SNR.
- a system having the characteristics of FIG. 1 adapts locally to the image content as measured by the gradient magnitude and globally to the input SNR of the image.
- FIG. 2 there is shown a simplified block diagram of a video enhancement circuit according to the present invention which approximates the characteristics shown in FIG. 1.
- the performance characteristics of the circuit of FIG. 2 are shown in FIG. 3.
- a television video signal is applied to the input of the enhancement circuit.
- An analog-to-digital converter 10 quantizes the signal into a plurality of successive digitized samples or pixels.
- the signal amplitude represented by each pixel is defined as x(i,j), where i is the number of the row or line during one complete frame of the video image and j is the number of successive samples in one line.
- At least two complete rows are temporarily stored in a two-line store 12, which may be a shift register. As each new sample is transferred into the register 12, a sample from at least two rows above in the image is shifted out of the register.
- a median sample m(i,j) is generated by a median filter 14.
- the median filter looks at a group of samples in the store 12 and selects the sample point of the group which has the middle magnitude.
- the median filter 14 looks at five image points or pixels arranged in a cross pattern, that is, three successive points in the same row or line, and three vertical points with the center point being common to both the horizontal and vertical lines.
- a suitable circuit for the median filter 14 is described below in detail in connection with FIG. 6.
- an average filter 16 which generates an output sample that is the average of a group or block of the samples stored in the store 12.
- Various averaging patterns can be used, such as a running line average, an area average, or a more complex averaging arrangement, such as described in the above-identified patent application.
- the samples stored may correspond to a 3 ⁇ 3 pattern of image pixels. To simplify division in obtaining the average value, only eight samples are used instead of nine. This allows a division by eight, which in a binary system only requires a three digit left shift operation.
- the resulting output sample from the average filter 16 is designated f(i,j).
- the magnitude of the gradient at any point on the picture can be determined by the difference in the median sample value m(i,j) and the average f(i,j).
- the sample gradient is obtained by a subtracting circuit 18 which subtracts the digitized output from the average filter 16 from the digitized output of the median filter 14.
- the gradient magnitude provided by the output of the subtracter is used to activate one of five processing units P 1 , P 2 , P 3 , P 4 or P 5 by comparing the gradient magnitude with a set of threshold values T 1 , T 2 , T 3 and T 4 derived from a table of threshold values stored in a threshold memory 20. Any one of a plurality of sets of four threshold values are selected, depending on the signal-to-noise ratio (SNR) of the video signal as determined by a noise monitor circuit 22.
- SNR signal-to-noise ratio
- the noise monitor circuit 22 as hereinafter described in detail, generates a digitized value which is proportional to the SNR of the video input signal.
- the noise monitor circuit utilizes the equalizing pulses of a received television signal as a reference, since the waveshape of the pulses are standardized. Any noise in the received signal can be detected as distortion of the equalizing pulse waveform.
- a noise monitoring system is described, for example, in U.S. Pat. No. 4,044,381.
- the output of the noise monitor circuit 22 is used to address the memory 20 to select a corresponding set of four threshold values from the table of thresholds stored in the memory 20.
- of the gradient magnitude is compared with the first threshold value T 1 by a comparator circuit 24. If the comparator indicates that the gradient value
- the processor P 1 is a simple gate circuit which is activated by the output C 1 from the comparator circuit 24 to gate the average sample from the average filter 16 to a common terminal 26 connected to the input of a digital-to-analog converter 28. Thus the average value sample f(i,j) is substituted for the input sample x(i,j) in generating an enhanced TV video signal.
- of the gradient is also compared by a comparator 30 with a threshold value T 2 which is larger than the value T 1 . If the gradient value is equal to or greater than the threshold value T 2 , the output of the comparator C 2 activates the processor P 2 .
- the processor P 2 in response to the output m(i,j) of the median filter 14, the output f(i,j) of the average filter 16, the absolute value
- the processor P 2 for carrying out the function of the equation 1 is shown in FIG. 7.
- of the gradient is scaled by the noise factor ⁇ ( ⁇ ) by a multiplier 32.
- the output is then multiplied by the median value m(i,j) by a multiplier 34.
- the output of the multiplier 34 is added to the average sample value f(i,j) by an adder 36 and applied to one input of a divider circuit 38.
- the output of the multiplier 32 is increased by one by an "add-one" circuit 40 and used as the divisor for the divider circuit 38.
- the output of the divider circuit 38 corresponds to the value P f (i,j) and is gated to the common terminal 26 by a gate 42 in response to the control signal C 2 from the comparator 30 when the gradient magnitude is greater than T 1 but less than T 2 .
- a comparator 44 turns on a control signal C 3 , activating the processor P 3 .
- the processor P 3 is merely a gate for gating the median value m(i,j) to the common terminal 26.
- a comparator 46 turns on a control signal C 4 which in turn activates the processor P 4 having the following characteristic:
- the processor P 3 is shown in FIG. 8.
- the processor P 4 again multiplies the absolute value
- the output of the multiplier 47 is added to the median value m(i,j) by an adder 48 to produce the desired edge enhanced value P e (i,j).
- This modified sample value is gated to the common terminal 26 through a gate 50 in response to the control signal C 4 .
- the processor P 5 modifies the median value m i ,j by adding or subtracting a constant value C and applying the output to the common terminal 26 in response to a control signal C 5 from the comparator 46.
- FIG. 3 The characteristics of the circuit of FIG. 2 are shown in FIG. 3. It will be seen that the characteristic of FIG. 3 is a close approximation to the desired characteristic of FIG. 1. From FIG. 3 it will be seen that if the magnitude of the gradient is below T 1 , the output sample corresponding to the average value f(i,j). Between T 1 and T 2 , the adaptive noise filter sample value P f is used, which reduces the degree of noise filtering as the magnitude of the gradient increases. Once the magnitude of the gradient exceeds the threshold T 2 , the output sample of the median filter is used as the output sample, resulting in less noise filtering but improved edge detail.
- FIG. 3 shows two different sets of thresholds, T and T' , corresponding to two different levels of noise, ⁇ 1 and ⁇ 2 .
- FIG. 4 A preferred embodiment is shown in FIG. 4 in which a single processor P 6 provides a modified sample value that provides both noise filtering and edge enhancement of the resulting image.
- the circuit is otherwise the same as the arrangement in FIG. 2 except that the threshold memory provides only two threshold values, T 1 and T 4 .
- Processor P 6 is activated between the full gradient range from T 1 to T 4 .
- the processor P 6 provides a combined noise filtering and edge enhancement according to the following relation: ##EQU2##
- of the gradient acts as a non-linear adaptive gain factor which changes P(i,j) from P f (i,j) to P e (i,j) smoothly as the gradient increases. Since T( ⁇ ) increases as ⁇ increases, this gain factor is de-emphasized as the input picture gets noisier.
- FIG. 9 shows a block diagram of processor P 6 .
- a median filter is shown in the block diagram of FIG. 6.
- the five samples from the cross-shaped block designated x 0 , x 1 , x 2 , x 3 and x 4 , are applied to a group of comparator circuits 50 from the store 12.
- Ten comparison circuits are required to compare each of the five samples with each of the other samples.
- Each comparator provides a binary coded output in which 0 represents less than and 1 represents greater than or equal.
- the ten binary bits from the ten comparators are applied to a decision logic circuit 52 which determines which of the five input samples is in the middle of the five samples.
- the decision logic sets a switch to connect the median value of the input samples to an output register 56.
- the concept of a median filter has been described in the literature. See, for example, “Nonlinear (Nonsuperposable) Methods for Smoothing Data" by J. W. Tukey, Comp. Rec., 1974 EASCON, p. 673.
- the averaging filter in its simplest form, sums a group of the stored points, preferably arranged in a block, and divides by the number of points to generate an average value.
- An alternative averaging circuit is described in copending application Ser. No. 133,606 filed Mar. 24, 1980, entitled “Adaptive Enhancement of Signal-to-Noise Ratio in Television Imagery” by Curtis May and assigned to the same assignee as the present application.
- a circuit which provides both noise filtering and edge enhancement in the same image.
- the degree of noise filtering and edge enhancement adapts locally to image content and globally to the SNR of the image.
- the averaging filter and the median filter can be tailored depending on application areas, giving the system considerable flexibility. Minimum storage is required and real-time operation for a TV system is no problem.
Abstract
Description
P.sub.e (i,j)=m(i,j)+β(σ)G(i,j);|G(i,j)|>T(σ)
Claims (9)
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US06/173,005 US4334244A (en) | 1980-07-28 | 1980-07-28 | Adaptive image enhancement system |
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Cited By (52)
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US4388646A (en) * | 1981-06-04 | 1983-06-14 | Rca Corporation | Low-distortion detection of pulses superimposed on an unknown and variable background signal |
US4395732A (en) * | 1981-08-19 | 1983-07-26 | Motorola Inc. | Statistically adaptive analog to digital converter |
EP0105619A2 (en) * | 1982-09-07 | 1984-04-18 | The Board Of Trustees Of The Leland Stanford Junior University | Multiple-measurement noise-reducing system |
EP0113433A2 (en) * | 1982-12-06 | 1984-07-18 | Kabushiki Kaisha Toshiba | X-ray image data processing apparatus |
US4472733A (en) * | 1982-09-01 | 1984-09-18 | Rca Corporation | Color channel signal-to-noise improvement in digital television |
DE3427669A1 (en) * | 1983-07-27 | 1985-02-07 | Rca Corp., New York, N.Y. | CIRCUIT ARRANGEMENT FOR IMPROVING SIGNAL TRANSITIONS |
US4513440A (en) * | 1982-06-29 | 1985-04-23 | Harris Corporation | Hardware median filter |
US4553042A (en) * | 1983-07-27 | 1985-11-12 | Rca Corporation | Signal transition enhancement circuit |
US4561022A (en) * | 1983-08-11 | 1985-12-24 | Eastman Kodak Company | Image processing method based on processing of interrelated image gradients |
US4571635A (en) * | 1984-02-17 | 1986-02-18 | Minnesota Mining And Manufacturing Company | Method of image enhancement by raster scanning |
US4587448A (en) * | 1983-07-27 | 1986-05-06 | Rca Corporation | Signal transition detection circuit |
US4595958A (en) * | 1984-08-24 | 1986-06-17 | Minnesota Mining And Manufacturing Company | Multiformat image recordation |
US4642689A (en) * | 1983-12-23 | 1987-02-10 | U. S. Philips Corporation | Increasing the resolution of a digitized, time-dependent signal |
US4736439A (en) * | 1985-05-24 | 1988-04-05 | The United States Of America As Represented By The Secretary Of The Navy | Image preprocessing by modified median filter |
US4782389A (en) * | 1987-04-30 | 1988-11-01 | Rca Licensing Corporation | Adaptive M-tile sample producer |
US4785347A (en) * | 1983-06-20 | 1988-11-15 | Fuji Photo Film Co., Ltd. | Method for emphasizing sharpness of a picture image by forming and processing sharp and unsharp signals from a picture image signal |
US4829381A (en) * | 1988-04-18 | 1989-05-09 | Polaroid Corporation | System and method for electronic image enhancement by dynamic pixel transformation |
US4933766A (en) * | 1988-06-02 | 1990-06-12 | U.S. Philips Corporation | Interpolation filter and receiver provided with such an interpolation filter |
US4941190A (en) * | 1988-07-15 | 1990-07-10 | Minnesota Mining And Manufacturing Company | Method and system for enhancement of a digitized image |
US5038388A (en) * | 1989-05-15 | 1991-08-06 | Polaroid Corporation | Method for adaptively sharpening electronic images |
EP0529903A2 (en) * | 1991-08-23 | 1993-03-03 | Mitsubishi Denki Kabushiki Kaisha | Image processing system |
EP0588181A1 (en) * | 1992-09-14 | 1994-03-23 | THOMSON multimedia | Method and apparatus for noise reduction |
US5379074A (en) * | 1992-07-18 | 1995-01-03 | Samsung Electronics Co., Ltd. | Multilevel nonlinear filter for edge detection and noise suppression |
EP0661549A1 (en) * | 1993-12-30 | 1995-07-05 | Corning Incorporated | Noise reduction of electrical signals |
EP0683605A2 (en) * | 1994-05-18 | 1995-11-22 | NOKIA TECHNOLOGY GmbH | Method and device for enhancing the transients of a video signal in component form |
US5490094A (en) * | 1992-09-14 | 1996-02-06 | Thomson Consumer Electronics, S.A. | Method and apparatus for noise reduction |
EP0762746A2 (en) * | 1995-09-06 | 1997-03-12 | HE HOLDINGS, INC. dba HUGHES ELECTRONICS | Thermal imaging device |
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WO2001072032A1 (en) * | 2000-03-24 | 2001-09-27 | Koninklijke Philips Electronics N.V. | N-dimensional filter and method for n-dimensionally filtering an original image pixel |
US20020186890A1 (en) * | 2001-05-03 | 2002-12-12 | Ming-Chieh Lee | Dynamic filtering for lossy compression |
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Cited By (78)
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US4388646A (en) * | 1981-06-04 | 1983-06-14 | Rca Corporation | Low-distortion detection of pulses superimposed on an unknown and variable background signal |
US4395732A (en) * | 1981-08-19 | 1983-07-26 | Motorola Inc. | Statistically adaptive analog to digital converter |
US4513440A (en) * | 1982-06-29 | 1985-04-23 | Harris Corporation | Hardware median filter |
US4472733A (en) * | 1982-09-01 | 1984-09-18 | Rca Corporation | Color channel signal-to-noise improvement in digital television |
EP0105619A2 (en) * | 1982-09-07 | 1984-04-18 | The Board Of Trustees Of The Leland Stanford Junior University | Multiple-measurement noise-reducing system |
EP0105619A3 (en) * | 1982-09-07 | 1987-08-26 | The Board Of Trustees Of The Leland Stanford Junior University | Multiple-measurement noise-reducing system |
EP0113433A2 (en) * | 1982-12-06 | 1984-07-18 | Kabushiki Kaisha Toshiba | X-ray image data processing apparatus |
EP0113433A3 (en) * | 1982-12-06 | 1984-09-12 | Kabushiki Kaisha Toshiba | X-ray image data processing apparatus |
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US4785347A (en) * | 1983-06-20 | 1988-11-15 | Fuji Photo Film Co., Ltd. | Method for emphasizing sharpness of a picture image by forming and processing sharp and unsharp signals from a picture image signal |
DE3427669A1 (en) * | 1983-07-27 | 1985-02-07 | Rca Corp., New York, N.Y. | CIRCUIT ARRANGEMENT FOR IMPROVING SIGNAL TRANSITIONS |
US4553042A (en) * | 1983-07-27 | 1985-11-12 | Rca Corporation | Signal transition enhancement circuit |
US4587448A (en) * | 1983-07-27 | 1986-05-06 | Rca Corporation | Signal transition detection circuit |
FR2557410A1 (en) * | 1983-07-27 | 1985-06-28 | Rca Corp | SIGNAL PROCESSING CIRCUIT |
US4561022A (en) * | 1983-08-11 | 1985-12-24 | Eastman Kodak Company | Image processing method based on processing of interrelated image gradients |
US4642689A (en) * | 1983-12-23 | 1987-02-10 | U. S. Philips Corporation | Increasing the resolution of a digitized, time-dependent signal |
US4571635A (en) * | 1984-02-17 | 1986-02-18 | Minnesota Mining And Manufacturing Company | Method of image enhancement by raster scanning |
US4595958A (en) * | 1984-08-24 | 1986-06-17 | Minnesota Mining And Manufacturing Company | Multiformat image recordation |
US4736439A (en) * | 1985-05-24 | 1988-04-05 | The United States Of America As Represented By The Secretary Of The Navy | Image preprocessing by modified median filter |
US4782389A (en) * | 1987-04-30 | 1988-11-01 | Rca Licensing Corporation | Adaptive M-tile sample producer |
US4829381A (en) * | 1988-04-18 | 1989-05-09 | Polaroid Corporation | System and method for electronic image enhancement by dynamic pixel transformation |
US4933766A (en) * | 1988-06-02 | 1990-06-12 | U.S. Philips Corporation | Interpolation filter and receiver provided with such an interpolation filter |
US4941190A (en) * | 1988-07-15 | 1990-07-10 | Minnesota Mining And Manufacturing Company | Method and system for enhancement of a digitized image |
US5038388A (en) * | 1989-05-15 | 1991-08-06 | Polaroid Corporation | Method for adaptively sharpening electronic images |
EP0529903A3 (en) * | 1991-08-23 | 1995-03-22 | Mitsubishi Electric Corp | Image processing system |
US5920654A (en) * | 1991-08-23 | 1999-07-06 | Mitsubishi Denki Kabushiki Kaisha | Image processing system that includes discrimination of an interpolation direction |
EP0529903A2 (en) * | 1991-08-23 | 1993-03-03 | Mitsubishi Denki Kabushiki Kaisha | Image processing system |
US5550936A (en) * | 1991-08-23 | 1996-08-27 | Mitsubishi Denki Kabushiki Kaisha | Image processing system |
US5379074A (en) * | 1992-07-18 | 1995-01-03 | Samsung Electronics Co., Ltd. | Multilevel nonlinear filter for edge detection and noise suppression |
GB2269071B (en) * | 1992-07-18 | 1996-02-28 | Samsung Electronics Co Ltd | Noise suppression in image signals |
EP0588181A1 (en) * | 1992-09-14 | 1994-03-23 | THOMSON multimedia | Method and apparatus for noise reduction |
US5490094A (en) * | 1992-09-14 | 1996-02-06 | Thomson Consumer Electronics, S.A. | Method and apparatus for noise reduction |
US8072539B1 (en) | 1993-07-26 | 2011-12-06 | Cooper J Carl | Apparatus and method for digital processing of analog television signals |
EP0661549A1 (en) * | 1993-12-30 | 1995-07-05 | Corning Incorporated | Noise reduction of electrical signals |
EP0683605A2 (en) * | 1994-05-18 | 1995-11-22 | NOKIA TECHNOLOGY GmbH | Method and device for enhancing the transients of a video signal in component form |
EP0683605A3 (en) * | 1994-05-18 | 1996-11-06 | Nokia Technology Gmbh | Method and device for enhancing the transients of a video signal in component form. |
EP0762746A3 (en) * | 1995-09-06 | 1999-06-09 | Raytheon Company | Thermal imaging device |
EP0762746A2 (en) * | 1995-09-06 | 1997-03-12 | HE HOLDINGS, INC. dba HUGHES ELECTRONICS | Thermal imaging device |
US6028957A (en) * | 1996-03-07 | 2000-02-22 | Minolta Co., Ltd. | Image forming apparatus having a noise removing unit |
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